Macrofossil assemblages in Canada show two biotic provinces during the Cretaceous: the North Pacific Biotic and the North American Boreal provinces. Cretaceous foraminiferal assemblages also attest to two different zoogeographic systems indicating separate paleoecological regimes divided by the emerging Cordilleran fold-and thrust belt. Microfauna in the foreland basin experienced a major faunal changeover at the end of the Albian associated with a global sea-level fall, oceanic anoxic event and hot greenhouse conditions. Widespread late Albian to early Cenomanian erosion is particularly evident throughout northwest and Arctic Canada where Boreal marine influence to the seaway temporarily closed. Subsequent sea-level rises throughout the Cenomanian brought Tethyan-derived benthic and planktic elements into the seaway. These Late Cretaceous foreland basin faunas differ from those in West Coast fore-arc basins where distal shales interfinger with turbidite, and coarse-grained submarine fan facies. In both basin settings, regional temperature, salinity gradients and oxygen related trophic levels formed barriers to faunal dispersal. The fore-arc basins experienced open ocean conditions that supported rich macrofossil, radiolarian and planktic foraminiferal assemblages. In the foreland basin, diverse outer shelf benthic foraminifera indicate increased salinities for the Santonian to Campanian, supporting the epeiric extension to Hudson Bay. Faunal similarities between foreland basin faunas and the Atlantic assemblage are evidence of pathways through the Hudson and Boreal seaways.